This invention relates generally to semiconductor memories.
Flash memories may be read a number of times. Each time the flash memory is read, its threshold voltage increases. After a number of read cycles, the threshold voltage may eventually become so high that a read failure occurs. Such a failure is called a read disturb.
In a read disturb, the cell fails to read and there is no practical way to access the stored information. As a result, the memory may be considered unreliable and the memory is essentially worthless even though the read disturb may be confined to one or only a few number of cells. In other words, the bulk of the memory array may be fully operable, but a failure in a small percentage of the cells may make the entire integrated circuit of no further value. The read disturb problem may be particularly severe when an isolated region of the array is read many more times than other regions.
Thus, there is a need for ways to monitor the threshold voltages of semiconductor memory cells.